A popular type of refrigerant compressor for use in automotive air conditioning systems includes a wobble or nutating drive mechanism. Such a compressor may either be designed for constant or infinitely variable displacement. In either design of this type, a plurality of cylinders are equally angularly spaced about a cylinder block and compressor housing, and equally radially spaced from the axis of a central drive shaft. A piston is mounted for reciprocating motion in each of the cylinders. A piston rod connects each piston to a non-rotatable socket plate of the wobble plate assembly. This socket plate provides the nutating motion in response to a drive plate that is attached to the drive shaft. The driving of the socket plate in a nutating path serves to impart the linear reciprocating motion to the pistons, thereby providing proper compressor operation.
The drive plate includes a recessed journal that receives a bushing by which the socket plate is held in place. The drive plate also includes a thrust bearing of the needle type mounted in the journal to reduce the friction at the rotary interface of the journal. The angle of both the socket plate and the drive plate with the thrust bearing in between is always the same relative to the drive shaft. The stroke of the pistons, and accordingly the displacement or capacity of the compressor, is in effect a function of this angle. As the drive plate rotates, the bushing and thrust bearing allow the socket plate to be held against rotation, but forced to travel in the nutating path.
In variable displacement wobble plate compressors, the angle of the socket plate and the associated thrust bearing and bushing is adjustable to control the displacement or capacity of the compressor. When the axis of the socket plate is substantially coincident with the axis of the central drive shaft, the compressor is operating at zero compression or zero stroke. When the axis of the socket plate is at its extreme allowable angle relative to the axis of the drive shaft, the compressor is operating at full compression or full stroke. It can be further appreciated that the stroke of the pistons is infinitely adjustable between the zero stroke and full stroke positions, as the socket plate pivots from the axial coincidence with the drive shaft to the extreme position.
In order to provide for ease of compressor assembly, it has long been known to retain the socket plate in position on the journal of the drive plate through the utilization of a snap-type retaining ring. As disclosed in, for example, U.S. Pat. No. 4,428,718 to Skinner, entitled "Variable Displacement Compressor Control Valve Arrangement", issued Jan. 31, 1984 and assigned to the assignee of the present invention, the inner peripheral edge or margin of the retaining ring is received in an annular groove or channel formed in the journal. The inside face of the outer peripheral edge of the retaining ring engages a thrust washer that is in juxtaposition with the associated bushing that holds the socket plate in place on the journal.
In past practice the retaining ring has included a formed angle on the outside face. This angle matches the angle of the mating side wall of the annular channel. Accordingly, when properly positioned, the retaining ring acts as a wedge to provide for a biasing function. This feature is intended to compensate for manufacturing tolerances to provide a proper fitting assembly. As a result, the bias force provided preloads the thrust washer and bearing against the journal. This results in smooth, chatterless operation, and a reduction in noise generated during high-speed rotation of the drive plate.
While this prior art approach does provide for ease of assembly and significant noise reduction during compressor operation, further improvement in this basic design is desired. More specifically, it has been found that the performance of the prior art retaining ring degrades slowly over time. This is particularly true in vehicles operated in geographical areas experiencing hot and humid weather conditions for extended periods of time. The air conditioning systems in such vehicles are often subjected to high load operating conditions that place significantly higher strain upon the compressor.
As the performance of the retaining ring degrades, a loss of preload force results. Eventually, the preload force drops to a level where sufficient biasing of the thrust washer against the journal is no longer provided to maintain noise free operation. This is a significant annoyance to the passengers in the vehicle as the noise is transmitted through the air conditioning system directly into the passenger compartment. Further, excessive play between the socket plate and the drive plate eventually develops. This promotes the generation of troublesome vibrations and accelerated wear that reduces the service life of the compressor.
Efforts to address the problem by adjusting the formed angle of the retaining ring and annular channel have met with only limited success. More particularly, some improvement in durability is provided but the problem still persists to a lesser degree. Accordingly, other approaches to improve the durability of the retainer assembly while maintaining the ease of production and assembly of component parts are needed, and even recognized by others. One such approach is disclosed in U.S. Pat. No. 4,820,132 to Higashahira et al. entitled "Variable Displacement Wobble Plate Type Compressor" issued on Apr. 11, 1989. In this design, a plate spring is provided to constantly bias the socket plate against the thrust bearing. The retainer/spring are rotatably locked into a biasing position. The plate spring, retainer and thrust washer are all required. The spring is received in a groove or channel in the journal of the drive plate, but is not directly engaged therein.
While this system is also effective, it too suffers certain drawbacks and is not the ultimate answer. More particularly, matching crenulations or arcuate projections and recesses must be formed in the various components of the retainer assembly to ensure that the retainer/spring does not vibrate out of the desired rotary position thereby leading to a drop in the preload force, production of noise and the presence of excessive play. Further, painstaking steps must be taken to ensure and confirm proper installation and the locking of the rotatable parts in position. Such time consuming steps are inconvenient and expensive. The need therefore exists for the present improved assembly for retaining the socket plate on the drive plate of a wobble plate compressor.